1. Field of the Invention
This invention relates a system for driving a drum, for example, of a color copying machine or a printer.
2. Prior Art
In recent years, it has been proposed to incorporate a pulse motor (stepping motor) in a drum of the type described so as to drive the drum for rotation. The pulse motor comprises an inner stator fixedly mounted on a drum shaft, and an outer rotor disposed around the inner rotor coaxially therewith and fixed relative to the drum, so that the drum is rotated together with the rotor. Since the pulse motor is accommodated within the drum, the machine can be of a compact size. The pulse motor of this type is often referred to as "outer rotor type" in the trade. This arrangement is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 49-115429. Generally, a pulse motor is subjected to a considerable torque ripple, and besides since the pulse motor is used in an open loop circuitry, the pulse motor is not rotated smoothly, which affects the quality of a resultant copied image or a printed image.
FIG. 1 shows a pulse motor 10 of the outer rotor type which comprises a hollow cylindrical rotor 12 disposed coaxially around a shaft 14 and having a number of rotor teeth T0 formed on an inner peripheral surface thereof and spaced a predetermined pitch, and a stator 16 fixedly mounted on the shaft 14 and disposed coaxially within the cylindrical rotor 12. The stator 16 has eight magnetic poles 18a to 18h spaced equally circumferentially, that is, at an interval of 45.degree.. Each of the magnetic poles 18a to 18h has a plurality of stator teeth T1 disposed in closed spaced opposed relation to the rotor teeth T0 of the rotor 12 and spaced from one another a pitch equal to the pitch of the rotor teeth T0 of the rotor 12. With this construction, when the stator teeth T1 are out of phase with the rotor teeth T0, as shown in FIG. 1, the stator teeth T1 of the magnetic poles 18b, 18c, 18d and 18e are 45.degree., 90.degree., 135.degree. and 180.degree. out of phase with the rotor teeth T0 of the rotor 12. Similarly, the stator teeth T1 of the magnetic poles 18f, 18g and 18h are 225.degree., 270.degree. and 315.degree. out of phase with the rotor teeth T0 of the rotor 12.
Exciting coils 20a to 20h are wound around the magnetic poles 18a to 18h of the stator 16, respectively. The coils 20a and 20e are serially connected together via a switch SW1, the coils 20b and 20f are serially connected together via a switch SW2, the coils 20c and 20g are connected together via a switch SW3, and coils 20d and 20h are serially connected together via a switch SW4. With this construction, when the switches SW1 to SW4 are sequentially closed, so that the mating coils 20a and 20e, the mating coils 20b and 20f, the mating coils 20c and 20g and the mating coils 20d and 20h are sequentially excited one after another, thereby rotating the rotor 12 in a clockwise direction in a step-like manner.
The position of the rotor teeth T0 of the rotor 12 is detected by either an optical sensor or a magnetic sensor, and in accordance with the results of this detection, the pulse motor 10 is supplied with current so as to serve as a servo motor. In this case, it is important that the cogging and the torque ripple are kept to a low level. To achieve this, it is necessary that the magnetic reluctance between the stator teeth T1 and the rotor teeth T0 varies sinusoidally. In other words, it is necessary that the voltage induced in each coil is sinusoidally varied in accordance with the rotation of the rotor 12 to reduce a distortion of the induced voltage. Conventionally, in order to achieve the above-mentioned sinusoidal variation of the magnetic reluctance or the induced voltage, grooves either of the rotor 12 or the stator 16 have the teeth separate from one another and are arranged obliquely relative to the axis of the shaft 14. Since each of the rotor 12 and the stator 16 is constructed of laminated sheets, it is rather cumbersome to provide such a skewed slot arrangement in the manufacture of the rotor or the stator. This problem is serious particularly where the pulse motor 10 is of the hybrid type including a rotor having a permanent magnet contained in the laminated core sheets.
In a drum drive system under consideration, generally, a detector called "resolver" is used for detecting the position of the magnetic poles of the pulse motor relative to the rotor of the pulse motor, that is to say, the position of rotation of the rotor, in order to energize the pulse motor at a proper timing. The conventional resolver comprises a rotor and a stator which are configured and arranged as described above for the conventional pulse motor, so as to cause the magnetic reluctance between the stator teeth T1 and the rotor teeth T0 to vary sinusoidally. Therefore, because of a skewed slot arrangement, the manufacture of such a conventional resolver is also cumbersome. Further, in such a magnetic pole position-detecting method, a two-phase sinusoidal exciting signal composed of sine and cosine waves is supplied to the stator coils of the resolver. In this case, it is known that the following formula is obtained: EQU X=A sin (2.pi.ft+.theta.)
wherein .theta. is an angle of rotation of the rotor, f is a frequency of the exciting signal, and X is an output of the stator coils. Therefore, X=A sin .theta. is obtained by sampling the output X every 2.pi.ft=2.pi., X=A cos .theta. is obtained by sampling the output X every 2.pi.ft=(2.pi.+/2), thereby detecting an angle of rotation of the rotor. However, there inevitably develops an error in amplitude and phase of the two-phase sinusoidal exciting current, and therefore the detection is not carried out accurately.